directed-attached storage (DAS)
In a DAS confi guration, disk drives or storage systems are connected directly to a server over SCSI or Fibre Channel. The host server formats the DAS using a compatible disk file system, such as UFS or HFS+, and shares the storage with clients over Ethernet using a network file system, such as AFP, SMB/CIFS, or NFS.

network-attached storage (NAS)
A NAS is a storage device with a built-in computer. This NAS appliance connects to a local area network and, like a DAS, shares storage with clients over Ethernet using a network file system. A NAS appliance typically features a specialized server operating system designed to make file-sharing setup and maintenance easier than with most general-purpose servers.

storage area network (SAN)
A storage area network, or SAN, is a method of aggregating storage devices and allowing servers and client computers to access them as a single virtual storage entity. The volume looks just like a DASlike a big hard driveto the servers. The difference is
that, with a SAN file system such as Xsan, all the servers can access the storage volume at the same time. These host servers can then share the data with additional clients on the local Ethernet network using a network fi le system, such as AFP, SMB/CIFS, or NFS.

Limitations of DAS and NAS systems: (1) Single point of failure; (2) Performance bottlenecks (all file systems must go through a single workstation or server); (3) Difficult scalability (limited by # of devices that can be attached to the server)

By Jason

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